A novelty search directed towards systems for detecting leaks in cryogenic storage tanks uncovered the following prior art:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ C. D. Forman et al 3,347,402 Oct. 17, 1967 N. K. Basile et al 3,413,840 Dec. 3, 1968 Basile et al 3,659,543 May 2, 1972 Katusuta et al 3,908,468 Sept. 30, 1975 Turner 3,919,855 Nov. 18, 1975 Oertle 4,104,906 Aug. 8, 1978 Peterson et al 4,135,386 Jan. 23, 1979 ______________________________________
The 1967 patent issued to Forman et al (U.S. Letters Patent No. 3,347,402) sets forth a cryogenic storage tank having a single insulation space pulled to a vacuum surrounded by insulating material. A monitor is placed in the primary insulating space which is adapted to detect the presence of cargo gas which would be indicative of a leak in the primary barrier.
The 1968 patent issued to Basile et al (U.S. Pat. No. 3,413,840) relates to a leak detection system for liquified gas storage tanks. The Basile leak detection system has for its goal the detection of leaks in the primary barrier of a cargo tank having a primary insulation space. The primary insulation space is filled with inert gas and is maintained at a pressure slightly less than the cargo gas pressure within the tank. The leak detection system of Basile senses both pressure and temperature in the primary insulation space and generates a calculated pressure which is then compared to the actual pressure expected. If a leak exists, the pressure in the primary insulation space will increase.
The 1972 patent issued to Basile et al (U.S. Pat. No. 3,659,543) sets forth a ship for transporting cryogenic material utilizing a cargo tank surrounded by a primary insulating space which is surrounded by the inner hull of the ship. Around the inner hull, is provided a cofferdam space. Inert gas is circulated in both the primary insulating space and the cofferdam space by means of two separate circulation systems of different inert gases. Leaks in either the primary barrier or the inner hull, will result in a leak of either the cargo gas or the inert gas which can be detected by means of a gas sensor or by a pressure buildup.
The 1975 patent issued to Katsuta et al (U.S. Letters Pat. No. 3,908,468) relates to a storage tank leak detector utilizing two electrically conductive layers creating a predetermined capacitance so that any leakage of the stored fluid will change the capacitance between the layers thereby indicating the leak.
The 1975 patent issued to Turner (U.S. Pat. No. 3,919,855) relates to the storing of an inert gas in the primary insulation space at a pressure sufficiently above atmospheric pressure to assure that any leakage occurring through the primary barrier or inner skin is only gas from the chamber to the contents in the tank and not vice versa.
The 1978 patent issued to Oertle (U.S. Pat. No. 4,104,906) relates to a crack detection arrangement utilizing a strain gauge sensor.
The 1979 patent issued to Peterson et al (U.S. Pat. No. 4,135,386) also relates to a system for monitoring early formation of cracks in permeable or porous materials.
In view of the prior art uncovered, two basic approaches for leak detection are presented. First, and as shown in the Forman et al patent, a gas monitor or detector is placed in an insulating space filled with an inert gas to detect any presence of the gas being stored in the cryogenic tank. If no leaks are present, no cargo gas would be present in the insulating space. Secondly, a differential pressure measurement can be made as set forth in the Basile et al U.S. Pat. No. (3,413,840) wherein the pressure of the inert gas in the insulation space is measured and kept slightly less than the cargo gas pressure within the tank. In the event that a leak occurs in the primary barrier, the pressure of the primary insulation space increases and this pressure, when corrected with temperature, is indicative of the leak.
For cryogenic storage tanks having primary and secondary insulation spaces, there is no known technique for detecting a leak in the secondary barrier where both insulating spaces utilize the same inert gas. For example, placing a gas monitor in the secondary insulation space would only detect the cargo gas in the highly improbable event of a leak occurring in both the primary and secondary barriers. Such a monitor would not detect a leak in the secondary barrier if no leak existed in the primary barrier. The use of two different inert gases are suggested by Basile et al, resulting in a costly approach requiring two sources of supply of inert gas.
To date, the only practical procedure for detecting leaks in the secondary barrier is to empty the storage tank, by removing the liquified natural gas, warming the storage tank up, and pulling a vacuum on the secondary insulation space. For a conventional tanker, having six cargo tanks, this process of verifying the existence of leaks and repairing those leaks takes from seven to ten days at a considerable cost in down time. Furthermore, such dry-docking inspections run the risk of causing leaks to occur in the primary and secondary barriers.
Finally, none of the prior art sets forth an approach for determining the approximate size of a leak.